In the field of medical devices which are implanted within the body of a human patient, such as implantable cardiac stimulators and the like, it is often desirable that certain operational parameters of the device be altered in a non-invasive (i.e. non-surgical) means. Various means of non-invasive communication with implanted devices have been previously disclosed. Since the introduction of demand cardiac pacemakers, one of the most common methods of non-invasive alteration of operational parameters employs a magnetically actuated reed switch contained within the implanted devices. The magnetic reed switch consists of a hermetically sealed cylindrical encapsulation housing two metallic reeds. The metallic reeds are disposed within the encapsulation such that when a sufficiently strong magnetic force is applied to the implanted device from outside the patient's body, the magnetic force draws the two reeds into electrical contact with one another, thereby completing an electrical circuit. When the magnetic field is withdrawn, the reeds separate, breaking the electrical circuit. Such an arrangement is disclosed, for example, in U.S. Pat. No. 3,805,796 entitled "Implantable Cardiac Pacer Having Adjustable Parameters" issued to Terry, Jr. et al. on Apr. 23, 1974, U.S. Pat. No. 3,920,005, entitled "Evaluation System for Cardiac Stimulators" issued to Gombrich et al. on Nov. 18, 1975, and U.S. Pat. No. 4,066,086 entitled "Programmable Body Stimulator" issued to Alferness et al. on Jan. 3, 1978, each assigned to the assignee of the present invention.
Reed switch closure is used to enable asynchronous operation for follow-up and trans-telephonic evaluation of the implanted pacemaker. In addition, rate and mode changes which occur upon reed switch closure are used to indicate device function and battery status. More recently, external devices have been developed which communicate with implanted devices via radio-frequency (RF) telemetry. An RF telemetry link allows two-way communication between an external device and an implanted device, and enables a wider range of operational parameters to be externally programmed. However, the use of an RF link does not necessarily eliminate the need for a magnetically-actuated reed switch. In U.S. Pat. No. 4,250,833 entitled "Digital Cardiac Pacemaker With Refractory, Reversion and Sense Reset Means" issued to David L. Thompson on Feb. 17, 1981, for example, the disclosed pacemaker cannot receive and process external RF telemetry signals until the reed switch is closed. Such an arrangement ensures that the implanted device will not be unintentionally re-programmed by extraneous RF signals to which the patient may be exposed.
Although magnetic reed switches are commonly employed in implanted devices, there are nonetheless several known problems associated with them. Reed switches are typically the only mechanical devices with moving parts in a pacemaker, making them more susceptible than the pacemaker's electronic components to damage or mechanical failure such as might result from vibration or mechanical shock. Although the glass encapsulation affords some measure of protection to the reeds, the capsule itself is susceptible to breakage. Furthermore, advances in electronic technology have resulted in progressively smaller pacemakers, so that the reed switch itself must be made very small, thereby adding to its fragility. Also, thin pacemakers, typically on the order of six to eight millimeters thick, prevent reed switches from being oriented in a preferred orthogonal orientation to the large flat surface area of the pacemaker case. Failure of the reed switch is particularly undesirable in the context of implantable devices, since replacement of such devices involves a surgical procedure.
While a reed switch must be sensitive enough to be responsive to an externally applied magnetic field, it is important that the switch not be so sensitive that it is responsive to every magnetic field to which the patient may be exposed in daily activity. As a result, the manufacturing tolerances for reed switches are low, making manufacturing costs high.
One attempt to overcome the disadvantages of a reed switch has been described in U.S. Pat. No. 3,766,928 issued to Goldberg et al. This patent discloses a potentiometer which is affixed to a small diametrically magnetized disc magnet. A second magnet is rotated outside the patient to cause the disc magnet to rotate and turn the potentiometer. This method itself has numerous disadvantages including the fact that it is also mechanical, and very small, and thus prone to breakage. In addition, the manipulation of the second magnet in order to adjust the potentiometer is more difficult and complex than the manipulation of the magnet required to actuate a reed switch.
Another attempt to overcome the disadvantages of a reed switch has been described in U.S. Pat. No. 4,301,804 entitled "Pacemaker With Hall Effect Externally Controlled Switch" issued on Nov. 24, 1981 to Thompson et al. and assigned to the assignee of the present invention. This patent discloses a pacemaker in which a circuit produces a strobe signal which is used to turn on a current flow through a Hall effect element once each pacemaker pulse cycle for a selected period of time. The presence of an external magnetic field alters the electrical properties of the Hall effect element (typically implemented in a bipolar integrated circuit fabrication process), so that a positive voltage is provided to the pacemaker circuitry when the element is strobed. While the Hall effect element is not a mechanical device, and is in that respect preferable to a reed'switch, the Hall effect element has proven to be less sensitive than a reed switch, requires expensive processing and packaging, and is not compatible with standard linear CMOS processing which is preferentially used in implantable medical devices.
As an alternative to using mechanical reed-switches or Hall effect elements to detect and measure magnetic fields, it has been proposed in the prior art to employ split-drain field-effect transistors, sometimes called MAGFETs, for this purpose. Although similar to a conventional field-effect transistor (FET), the drain of a MAGFET is split into two isolated halves. Application of a magnetic field to a MAGFET device gives rise to a differential in the currents in the two split-drain-halves, the extent of this differential being directly proportional to the strength of the applied magnetic field. Although MAGFETs, like Hall-effect devices, have the advantage of being solid-state devices, some problems with prior MAGFETs are known. One problem relates to the sensitivity of these devices to magnetic fields. Typically, the gain constant of a MAGFET is small, i.e. the current differential between the drain current in respective drain-halves is rather slight for a given change in magnetic field intensity. In order for a MAGFET to be readily utilized to detect and measure an external magnetic field, it is desirable to maximize the gain constant of the MAGFET. It is has been taught in the prior art that the gain constant of a MAGFET can be increased by raising the bias current of the MAGFET in steady-state. (See, e.g., Misra et al., "A Novel High Gain MOS Magnetic Field Sensor", Sensors and Actuators, pp. 213-221 (1986)). In addition, many prior art references discuss operation of MAGFETs with five- to ten-volt power supplies and 10- to 620-micro-amp bias currents. (See, e.g., Misra, "A Novel CMOS Magnetic Field Sensor Array", IEEE Journal of Solid-State Circuits, v. 25, no. 2, April 1990, pp. 623-625; Nathan et al., "Design of a CMOS Oscillator with Magnetic-Field Frequency Modulation", IEEE Journal of Solid-State Circuits, v. SC-22, n. 2, April 1987, pp. 230-232.) Such power supply voltages and bias currents are not available in implantable pacemakers, however, which must operate with one- to three-volt power supplies and ten to one-hundred nano-amp bias currents.
Another method of the prior art for achieving increased sensitivity to magnetic fields involves using an array of MAGFET devices in a tree arrangement, and combining the drain-halves from the individual MAGFETs such that the effect of the magnetic field is measured by the additive effect of the current differential in each of the individual split-drain pairs. (See, e.g., Misra et al., "A Novel High Gain MOS Magnetic Field Sensor", Sensors and Actuators, pp. 213-221 (1986).) This solution, however, requires a higher-level supply voltage, since the threshold voltages of MAGFETs coupled in this manner are additive. Moreover, the use of multiple MAGFET devices results in a corresponding increase in total circuit area, and an increase in current drain for the sensor. Thus, prior art techniques for increasing the gain constant of MAGFET devices have proven unsuitable for application in implantable medical devices, which must maintain small size, long-term stability and reliability, minimal power-supply voltage levels and minimal current drain.
It is accordingly a feature of the present invention that an implantable medical device is provided Which uses a non-mechanical sensor which is sufficiently sensitive to external magnetic fields.
It is another feature of the present invention that an implantable medical device using a magnetic sensor is operable at the low supply voltages and bias currents available in an implanted medical device.
It is still another feature of the present invention that an implantable medical device uses a magnetically-actuated device which is simple to produce using conventional CMOS processing techniques.
It is yet another feature of the present invention that an implantable medical device using a magnetically actuated sensor is sufficiently sensitive to external magnetic fields even when the sensor is provided with the low supply voltages and bias currents of an implantable medical device.